Magnetic tape servo tracking systems' accuracies continue to improve over time, where currently, the traditional servo error, PES (Position Error Signal), is no longer the most significant tracking error. Many tape servo and tracking technologies are able to demonstrate performance capability in the sub one micron for PES errors.
In examining the contribution of position errors to the total off-track budget, it is evident that core pitch and track pitch mismatch fast becoming the dominant limitation. Unfortunately, the pitch related errors are embedded into the multi-channel flexible tape's geometrical limitations, and it is well understood that traditional servo control have difficulties in correcting pitch related errors as caused by the read-head or media. For example, typical off-track component caused by pitch error is in the range of 1-2 microns, and pitch error is by far the largest component of off-track error in the next generation tape drives. While this error may be insignificant in a tape drive within an error budget in the range of 8-10 microns, the same error would be unacceptable for next generation products with much higher track density.
It is also important to note that advances in tape guide and improvements in servo technology as applied to current tape systems are fast approaching the point of diminishing return. For example, in a guiding and servo system that could reduce the position error by half, the benefit to the system is a little more than a fraction of a micron (e.g., 0.1-0.2), as only one core could benefit. However, the “mismatch” between the tape tracks and head cores is at least one order of magnitude higher. Therefore, in the next generation systems the track pitch error is a major limiting factor towards increasing track densities.
In a system with dedicated servo technologies that use “surrogate servo sensors”, such as optical or magnetic heads, the accuracy is further limited by the system's inability to observe this mismatch as seen by the various data transducers or head cores. Therefore, only the static portion of the error could be corrected. The system would not be able to accommodate the dynamic changes in track pitches along the length of the tape.
In addition, it is well known in the tape industry that mismatch between the read-head (e.g., head cores, etc.) and the multiple data tracks written on a given tape can be a significant contributing cause of off-track error. Typically mismatch can be caused by two factors. First, when data is interchanged between two drives, the respective heads introduce core pitch mismatch between the two written data sets. With the advances of thin film head technology, mismatch has been reduced in comparison with traditional ferrite heads designs. However, at ultra high track densities, such as 10K TPI (Tracks Per Inch) and beyond, this mismatch becomes very significant. The second mismatch error contributor, which can occur even at lower TPI, is the differences in the expansion characteristic between the head and tape media due to environmental changes. The media can expand significantly due to thermal and/or hydroscopic variations.
Therefore, there is a need for methods and apparatuses that permit a magnetic tape drive to compensate for the dynamic changes in track pitches on the magnetic data storage tapes. In particular, the ability to adaptively modify track pitches of the magnetic tape to match the transducers on a magnetic read-head can enhance the performance of tape drives that utilize magnetic tapes with high track density.